Biomass for biofuel production contains large amounts of xylan as well as smaller amounts of other matrix polysaccharides, such as xyloglucan, mannans, pectins, and the like. Efficient biofuel production requires efficient degradation of these polysaccharides. Enzymatic degradation into monosaccharides is hindered, however, by acetyl ester substitutions on the polysaccharide backbone. The inhibition of degradation by acetyl esters can be significant, as substitution can be at a high level, e.g., typically 25-50% of the xylose residues of grass xylans are acetylated. Moreover, the acetic acid that is released during enzymatic or chemical degradation is inhibitory to organisms, such as yeast, that are used for fermentation. The acetic acid contained in a biomass mixture for fermentation can easily be in the order of 0.1 M or 6 g/l, which is a highly inhibitory level. Reduction in the level of acetic acid would therefore be highly beneficial for fermentation. Accordingly, there is a need for improvement to biofuel production to reduce acetic acid levels.
A pathogenic fungus, Cryptococcus neoformans, has a polysaccharide coat consisting of O-acetylated glucuronoxylomannans. A protein encoded by the Cas1p gene has been identified as being essential for acetylation of the coat polysaccharide (Janbon et al., Molec. Microbiol. 42:453-467, 2001). Although the gene has been putatively annotated as an acetyltranserase, its biochemical activity was not confirmed. Homologs of this gene have been identified in various plants.
The present invention is based, in part, on the discovery that mutations in homologs to Cas1p reduce polysaccharide acetylation. Plants having reduced polysaccharide acetylation in accordance with the present invention can be used, for example, to provide plant mass that produces lower levels of acetic acid during fermentation.